Aussie Widget Measures Wood Pole Strength

Accurate MPT field test enables Georgia Power to safely extend life of pole fleet.

H. Stewart Martin, Georgia Power | Feb 01, 2009

T&D Poles are an Electric Utility's Greatest Single Infrastructure Investment. They represent one of the utility's biggest risks, as pole failure can seriously impact public safety and reliability. There has been no proven technique to provide an accepted empirical measure of the remaining strength of in-service poles — that is, until recently. After learning more about the benefits of mechanical pole testing (MPT), Georgia Power (Atlanta, Georgia, U.S.), a Southern Company, put this new type of inspection method to the test.

POLES AND INSPECTIONS

The distribution poles at Georgia Power are primarily of the Southern Pine species and are subjected to very hot and moist weather conditions. The utility's older poles — mostly pressure-treated creosote — normally begin to deteriorate below ground at about 20 to 25 years into their service life.

Georgia Power has had a robust inspection and treatment program in place since the late 1980s. Prior to 1987, the utility primarily used the hammer-sounding test as the initial means of identifying suspect poles — there was no remedial treatment program in place. All of Georgia Power's purchased poles have been supplier treated with chromated copper arsenate (CCA) preservative since the late 1980s. To date, the utility has seen no deterioration of properly manufactured and treated CCA poles.

In addition to the decay damage done to Georgia Power's creosote poles prior to 1987, many attachments have been added to the poles for telecommunications, Internet and cable TV equipment. This all adds to the horizontal and vertical loading of the poles. The additional loading must be accounted for and compared to the pole strength for in-service poles.

It is imperative that unserviceable poles be removed from the system or properly reinforced. However, it is just as important not to remove serviceable poles prematurely. The cost of pole replacements vary from US$400 to $10,000, depending on the complexity of the attachments and the equipment on the pole.

In recent years, Georgia Power was finding that pole inspection vendors were becoming increasingly conservative in their evaluation of poles to reduce their risk and that of the utility. Georgia Power pole replacement crews expressed to management that they were being asked to replace more poles that appeared to be sound than in previous years.

MECHANICAL POLE TESTING

Georgia Power's Distribution Design and Performance group, which handles the asset management guidelines for the distribution side of the business, recently decided to pilot and evaluate a new type of inspection method: the MPT 40. This process was developed by Deuar Pty Ltd. (Burpengary, Queensland, Australia). It was quite different than any of the traditional pole inspection methods used by most electric utilities in the United States.

Georgia Power began discussions with Dr. Kris Deuar in early 2006 to better understand the technology, safety issues and costs. The utility was initially concerned about the safety of these partial load tests, because it only would be testing weakened poles occasionally. It became convinced of the safety of the tests, as the weaker poles would be found with either a good visual and sounding inspection, or with only a minimal amount of force applied by the MPT device.

The MPT 40 approach made sense to Georgia Power. It gave a "direct" indication of the pole's strength, taking into account the differences inherent in the wood species used to produce the pole, the orientation of the defects and so forth. The theory is that by applying a known bending force, and then measuring very accurately how the pole geometry changes, the bending strength of the pole can be calculated. MPT had been used extensively in Australia, New Zealand and China with good reported success. Furthermore, the Forest Service Research Institute of New Zealand recommended it as the best method available for determining in-service pole strength.

The method uses digital protractors, attached to a pole, which measure the tilt (bending back) of the pole as the small pressure against the pole (always much less than the residual pole strength) is first applied and then released. Each pole is audio-visually inspected and subjected to a small initial load of 200 lb to 300 lb (91 kg to 136 kg) and then analyzed for safety before a final target load of 2000 lb to 3000 lb (907 kg to 1360 kg) is applied.

THE PILOT TEST EXPERIENCE

In late summer 2006, Georgia Power had two conventional inspection vendors set to inspect and treat poles in Savannah, Georgia. Each vendor was to inspect and treat half of Savannah's pole plant. The utility contracted with Deuar to come to Savannah and perform tests on 100 of these poles. Two segments of the Savannah poles were selected to compare the MPT methodology for assessing serviceability with that of each conventional inspection contractor. In each vendor's assigned area, 50 poles were first tested by MPT, then later by one of the two conventional inspection vendors who did not know the result of the MPT evaluation.

In many cases, the two approaches (conventional versus MPT) were in close agreement and resulted in the same pass/fail determination ("fail" was given to poles that were less than two-thirds of their original nominal strength). However, in many cases, there was quite a bit of difference in the percentage-strength determinations.

Tables 1, 2 and 3 summarize the results of the pilot. It is significant to note that:

Six poles that had been rejected by conventional inspections were rated by MPT as still serviceable.

Four poles that had been found to be still serviceable by conventional means were rated by MPT as unserviceable.

However, the question remained: Was the MPT evaluation more accurate or just different?

LABORATORY RESULTS

In an attempt to answer this question, Georgia Power joined an industry coalition in 2006 to perform pole tests with the National Electric Energy Testing, Research and Applications Center (NEETRAC). Several pole testing providers conducted independent analyses of the poles' remaining strength while they were still in-service. The poles were removed from service in 2007, and later break tested by NEETRAC in the lab.

Those tests proceeded slowly and were finally completed in the summer of 2007. The recently published report NEETRAC report showed the MPT process as one of the top-two predictors of pole strength. However, there were concerns about the useful application of the results. There was possible degradation of the poles over time and when they were removed and transported from the field location to Atlanta. Additionally, a great number of the poles failed at points well above the ground line, but every field vendor analysis addressed strength at ground line. Another series of tests is planned in 2009, where the test poles will be break tested in situ after the various vendors provide the predicted strength numbers to NEETRAC. Those involved believe that this will resolve the concerns of the previous tests.

ANOTHER ROUND OF TESTS

In December 2006, Georgia Power asked Deuar to test 10 poles in Atlanta, nine of which recently had been rejected (found to have less than 67% remaining strength) during a conventional evaluation. The utility's plan was to have Deuar test all of those poles using the partial load, nondestructive methodology. After completing those tests, Deuar would then use the more robust MPT 20 to break test these poles in situ.

Because the final series of tests was destructive, Georgia Power took precautions to ensure the safety of personnel and property. The utility's worries were put to rest during the break tests, as none of the poles failed in a way that required support of the pole. None fell over. At failure, the poles simply quit resisting the force of the MPT 20, the pressure dropped and the highest force was recorded to calculate the breaking strength.

The nondestructive round of tests, conducted with an MPT 40, calculated that eight of the nine poles previously rejected by the conventional evaluation were still serviceable and confirmed one as unserviceable. The MPT 40 test agreed with the conventional vendors on the one pole they found serviceable.

Georgia Power then had Deuar test the same 10 poles in situ, using an MPT 20, by applying force against them until they actually broke. These tests closely matched the MPT 40 findings, with eight poles still reflecting years of serviceable life and only one pole that had been found serviceable in the nondestructive test was found to be borderline reject in the destructive test (see the comparison of results in Table 4).

FORT GORDON TESTS

Although lacking an independent laboratory comparison test, Georgia Power nonetheless felt more confident seeing the reasonably close agreement of the nondestructive tests with the observed destructive tests. It also felt that the upcoming NEETRAC tests would further prove the worth and accuracy of the MPT technology. With this confirmation in hand, Georgia Power wanted to do additional testing. The late 2006 conventional inspection and treatment of poles in Fort Gordon and in the Atlanta operating area gave the utility an ideal opportunity.

The company had seen an above-average reject rate in Atlanta and Fort Gordon. The utility also knew how compelling the business case is for extending the life of a pole. Although it could not justify retesting all 50,000 poles in Atlanta, or all 4500 in Fort Gordon, Georgia Power knew it would only have to avoid replacing a small percentage of the reject poles with the MPT tests to make a good return on its investment.

Dr. Deuar was asked to test 234 rejected — and destined for replacement — poles in the Atlanta and Fort Gordon areas. All of these poles had been found unserviceable in early 2007 by conventional ground line inspection. Poles were selected for MPT that were high-cost replacement poles, those with either transformer banks, electrical junctions or other equipment that made replacement more expensive than simpler poles. Of the 234 conventionally rejected poles, 132 poles (56%) were evaluated by the MPT tests as being still serviceable.

Looking at the financial side of this approach, for its business case, Georgia Power established or assumed (historical records):

The average cost of replacement of one of these rejected poles was estimated to be around $4000.

The cost of testing each pole was approximately $200, which was relatively high as only a few widely scattered poles were chosen. Startup costs also were a big part, because all the men and equipment had to come from the other side of the globe for this project only. It is expected that these costs will come down as the process becomes more automated and the number of poles tested rises in a given cycle.

As a result, the cost savings were as follows:

Cost of pole testing 234 × $200 = $46,800

Cost saved on pole replacements 132 × $4000 = $528,000

Net savings $528,000 - $46,800 = $481,200.

The costs savings were all on the capital side of the financial analysis; the testing was an operating cost. Most utilities, Georgia Power included, regard these costs differently, but these savings are significant in any form of cash.

SAFETY IMPACT

From a safety standpoint, it also should be noted that out of 102 failed poles, the MPT found 21 poles (21%) to be much weaker than originally predicted by the conventional pole inspection methods. This allowed Georgia Power to place a higher priority on those poles that were previously thought to be low-priority replacements or reinforcements.

The traditional methods of testing a pole's strength — by hammering, listening to the pole's echo and boring — are recognized to be pretty unreliable. Most traditional pole testing methods assume consistent wood strength by species, age and remaining amount of good wood. Experience has shown these are false assumptions. Knowing a pole's species, age and degree of decay does not guarantee an accurate assessment of its remaining strength (or longevity). This knowledge can only be indicative of a pole's strength.

The initial stages of fungus growth, commonly known as an incipient decay, eludes all conventional methods of testing a pole's strength and, to date, can only be identified by costly microscopic examinations in a biological laboratory. It is not always detectable by drilling, yet incipient decay can reduce pole strength by up to 50%.

Additionally, more advanced internal decay or termite damage in a pole is often missed by drilling, especially if the pole cannot be fully excavated to inspect for belowground decay. Some Georgia Power poles had failed in-service due to belowground damage that had eluded inspectors.

AN EXCELLENT NEW TOOL

Georgia Power believes the recent field testing proves the MPT system is an excellent supplemental tool to conventional pole inspection and treatment methods. As the cost of the test is driven down by process improvements and higher volumes, it may even become more of a primary tool.

Although MPT cannot replace the remedial treatments performed by the traditional service providers, it could prevent the need to replace or reinforce poles that are either heavily loaded or found to have significant decay, rejected by conventional evaluations.

The business case is already convincing to support the use of MPT for performing a follow-up evaluation of poles rejected by the conventional inspection methods. For poles that a utility is unable to excavate, MPT also may be used to more accurately evaluate remaining strength, removing significant risk for the utility.

ACKNOWLEDGEMENT

The author thanks Dr. Kris Deuar for his assistance during these pilots and Troy M. Doyle of ONE Management Consulting Services, Dr. Deuar's North American representative.

H. Stewart Martin ([email protected]) began working at Georgia Power in 1968 as a cooperative student in distribution planning. After earning his BSME degree from the Georgia Institute of Technology, he held the positions of distribution engineer, operating supervisor for T&D construction and maintenance crews, engineering supervisor, staff engineer (primarily in metering), operating superintendent and power delivery manager, network distribution. In 1997, he returned to corporate distribution as the reliability project manager. He is now project manager, asset management. Martin is passionate about specifying the types of maintenance programs carried out at Georgia Power and has worked accident-free his entire career.